1. Technical Field
One or more embodiments of the present invention relate to a motor drive device including an inverter circuit driven by a PWM (Pulse Width Modulation) signal, and in particular, to a generation unit of a boost voltage for driving a semiconductor switching element for fail safe arranged between the inverter circuit and the motor.
2. Related Art
In an electrical power steering device of a vehicle, an electrical motor such as a three phase brushless motor is arranged to provide a steering assisting force corresponding to the steering torque of the handle to a steering mechanism. A motor drive device by the PWM control method is known for the device for driving the motor.
The motor drive device of the PWM control method generally includes an inverter circuit driven by a PWM signal having a predetermined duty. The inverter circuit is configured by a so-called three phase bridge in which three sets of a pair of upper and lower arms including a semiconductor element on the upper arm and the lower arm are arranged. The voltage of each phase is supplied from the inverter circuit to the motor to thereby drive the motor when each switching element carries out the ON/OFF operation based on the PWM signal.
A motor drive device in which a fail safe mechanical relay is arranged between the inverter circuit and the motor to prevent current from flowing from the inverter circuit to the motor (or from the motor to the inverter circuit) when failure of the circuit is detected is also known (e.g., Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746).
Such a failure of the circuit includes various failures that occur in the motor drive device. For instance, the failure may be an ON failure in which each switching element of the inverter circuit does not change from the ON state to the OFF state and remains in the ON state, or a short circuit failure that occurs at the wiring portion other than in the switching element. In addition, the failure may be such in which abnormality occurs inside the CPU that controls the inverter circuit and the CPU carries out a control different from the original control.
When such failure of the circuit is detected, a control in which the switching elements of the inverter circuit are all set to the OFF state or a control in which a power supply relay arranged between the inverter circuit and the vehicle battery is set to the OFF state is carried out. The power supply to the inverter circuit and the motor is thereby stopped, so that breakage of the inverter circuit, false operation such as assistance being carried out in the direction not predicable by the driver, or the like can be prevented.
When the failure of the circuit is detected, the driver steers the handle by human power because the steering assisting force by the motor is not applied. In this case, the handle and the motor mechanically interlock and turn in the electrical power steering device, thus enabling the assistance of the steering by the motor. Therefore, the motor rotates in cooperation with the steering of the handle and the motor functions as a power generator when steering with human power. Therefore, a large resistance applies on the handle operation by the power generating operation of the motor if the motor and the inverter circuit remain electrically connected. That is, a great amount of force will be required to turn the handle. The fail safe mechanical relay for electrically disconnecting the inverter circuit and the motor is thus installed as in Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746 to prevent such a drawback.
However, in the electrical power steering device, there is a demand to further miniaturize the control circuit although the supply power to the motor is large because the control circuit is to be mounted in the vehicle. Therefore, if the mechanical relay is used as in Japanese Patent Publication No. 3686471 and Japanese Unexamined Patent Publication No. 2005-199746, the relay itself becomes large and such a demand cannot be met. A motor drive device in which a fail safe semiconductor switching element is arranged between the inverter circuit and the motor in place of the mechanical relay as described in Japanese Patent Publication No. 3884450 is thus known.
In the motor drive device of patent document, an N-channel MOS-FET (Metal Oxide Semiconductor Field Effect Transistor) is arranged on a power supply line between the motor and the inverter circuit, where the MOS-FET of each power supply line is turned OFF when abnormality occurs to shield the motor from the inverter circuit.
When driving the N-channel MOS-FET, a high voltage in which the power supply voltage is boosted needs to be applied to the gate in the circuit configuration in which the potential of the source becomes substantially the same as the power supply voltage at the time of conduction (ON). Thus, in the motor drive device of Japanese Patent Publication No. 3884450, a booster circuit is arranged for every N-channel MOS-FET installed on each power supply line between the motor and the inverter circuit. The voltage boosted in the booster circuit is then supplied to the gate of the fail safe MOS-FET. However, in such configuration, the booster circuit for driving the fail safe MOS-FET needs to be independently arranged and the booster circuit needs to be arranged by the number of power supply lines, which complicates the circuit configuration.